According to the U.S. Department of Energy, 47% of the average home’s energy use is for heat—either domestic hot water or space heating. Using specially designed collectors, solar hot water (SHW) systems collect the sun’s heat and store it in water. In turn, this stored energy is tapped for domestic uses like showers, laundry, dish washing, or even heating your home.
Installing a solar hot water system represents one of the best renewable energy (RE) investments that the average homeowner can make. While wind and microhydro can have a lower cost per KWH equivalent of energy generated, only a small percentage of Americans live where they can install either of these technologies.
On the other hand, most home sites in the United States have at least some access to direct sunlight. Solar electricity—converting sunlight into electricity—is very popular. But compared to solar hot water collectors, photovoltaic modules are comparatively inefficient and expensive. The efficiency range for crystalline PV modules converting sunlight into electricity is about 12% to 19%, and recouping an investment in a grid-tied residential PV system, even with incentives, can take between ten and twenty years. In contrast, for domestic water heating applications, SHW collectors convert about 60% of the energy in sunlight into hot water, producing up to about six times more usable energy per dollar invested. In locations with favorable incentives, SHW systems can recoup their costs in about three to seven years.
Even without any available financial incentives, a professionally installed SHW system with 64 square feet of collector area can produce the equivalent of about 20 KWH per day at an installed cost of $5,000 to $8,000. If you wanted to generate that amount of energy with PV, a system size in the range of 3.5 KW would be required, costing approximately $20,000 before any financial incentives are applied.
Despite the obvious advantages, there has only been a modest increase in the number of SHW installations over the last several years compared to the significant growth in installed PV systems. With rising fuel costs and increasing interest in reducing carbon emissions, economic interests alone should be driving more significant growth in the solar thermal market than we are seeing. So what’s the holdup?
Solar hot water system installers are faced with myriad engineering decisions. Every home is different, and the optimal system type depends on a wide range of variables, including the number of people in the home, how efficiently water is used in the household, the average daily peak sun-hours available at the site, potential shading from nearby trees or buildings, wintertime ambient temperatures, and the location and space available for collectors and hot water storage.
Once the optimal system type—open loop, drainback, or glycol—is chosen, the installer needs to determine collector type (flat plate or evacuated tube), collector sizing, flow rates, pump performance, heat exchanger sizing, valve type, wiring, mounting, equipment sourcing—and decide how to integrate all the components together. Despite comparatively modest system costs compared to PV, technically complex system design and installation is one reason that many potential installers shy away from the SHW field.
Three common product packages in the European SHW market have rapidly streamlined solar thermal installations overseas and are gaining popularity in the United States.
The goal of each approach is to simplify system design, planning, and engineering, and to minimize system installation time and complexity.